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Kimberlite from Rajmahal Magmatic Province: Sr-Nd-Pb Isotopic Evidence for Kerguelen Plume Derived Magmas Anil Kumar, A

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Kimberlite from Rajmahal Magmatic Province: Sr-Nd-Pb Isotopic Evidence for Kerguelen Plume Derived Magmas Anil Kumar, A View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Publications of the IAS Fellows GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 20, 2053, doi:10.1029/2003GL018462, 2003 Kimberlite from Rajmahal magmatic province: Sr-Nd-Pb isotopic evidence for Kerguelen plume derived magmas Anil Kumar, A. M. Dayal, and V. M. Padmakumari National Geophysical Research Institute, Hyderabad, India Received 20 August 2003; accepted 15 September 2003; published 24 October 2003. [1] Previous studies showed that the Rajmahal-Sylhet- manifestations of decompression melting above a ‘hot cell’. Bengal (RSB) flood basalt province (117 ± 2 Ma) in eastern Comparison of geochemical and Sr-Nd-Pb isotopic compo- India was spatially close to the active Kerguelen hotspot sitions of Kerguelen plateau basalts with Rajmahal traps led about 118 Ma ago. Yet, it could not be unequivocally Mahoney et al. [1983] and Kent et al. [1997] to believe that correlated to this hotspot due to wide variation in isotopic the Kerguelen plume had not fed the Rajmahal magmas but compositions of both the RSB and Kerguelen plateau basalts. could have provided the heat source for their production. However, we report Sr-Nd-Pb isotopic compositions Storey et al. [1992] and Ingle et al. [2002], however, argued 87 86 ( Sr/ Sri: 0.70535 to 0.70561; eNd(T): À2.6 to À3.2; that this plume could also have furnished magmas for the 206 204 Pb/ Pbi: 17.88 to 18.07) of a co-eval (116 ± 2 Ma) Rajmahal lavas. Disagreement over the source of the RSB Group II kimberlite from this flood basalt province that is basalts is because their isotopic data is affected by variable identical to recently identified pristine Kerguelen plume amounts of contamination by a MORB like component in basalts from the Kerguelen Plateau/Archipelago and Broken Group I Rajmahal lavas and by crustal contamination in the Ridge. This suggests that the Kerguelen hotspot could indeed Group II lavas [Storey et al., 1992; Kent et al., 1997] as they be responsible for the 117 Ma magmatic activity in Eastern intruded a thick Precambrian continental crust. To overcome India. INDEX TERMS: 1025 Geochemistry: Composition of the this problem, a kimberlite intrusion, with high abundances mantle; 1040 Geochemistry: Isotopic composition/chemistry; 4825 of Nd > 250 ppm, Sr > 2200 ppm and Pb > 39 ppm, having Oceanography: Biological and Chemical: Geochemistry; 9340 temporal affinity (Ar/Ar ages between 109 and 117 Ma, Information Related to Geographic Region: Indian Ocean; 8450 Pringle et al. [1994]; Kent et al. [1998] with the RSB and Volcanology: Planetary volcanism (5480). Citation: Kumar, A., occurring to the southwest of the Rajmahal trap exposures A. M. Dayal, and V. M. Padmakumari, Kimberlite from Rajmahal (supplement-1a) has been studied for their age, Sr, Nd and magmatic province: Sr-Nd-Pb isotopic evidence for Kerguelen Pb isotopic compositions. These data are presented here and plume derived magmas, Geophys. Res. Lett., 30(20), 2053, compared with the Rajmahal basalts and the Kerguelen, doi:10.1029/2003GL018462, 2003. Crozet and Conrad hotspot lavas. 1. Introduction 2. Samples and Analytical Methods [2] A flood basalt province (Figure 1), consisting of the [3] Kimberlite samples used are from three boreholes Rajmahal, Sylhet and Bengal (RSB) traps (117 ± 2 Ma old; CMJP 32, CMJP 33, and CMJP 51, which are within about Baksi [1995]; Kent et al. [2002]; Pande and Ray [1999]) 3 km of each other and about 10 km southwest of Jharia occurs in eastern India (covering an area of 2 Â 105 km2). town. The samples consist of euhedral olivine phenocrysts, This province is part of a widespread Early Cretaceous pseudomorphed occasionally, macrocrystic and phenocrystic magmatism along the Indian, Australian/Antarctic margins phlogopite, microphenocrystic diopside, apatite, opaques, in [Kent et al., 2002]. Episodes of such large volcanism often a groundmass of k-feldspar, phlogopite, carbonates, perov- mark the initiation of sub-continental hot spots, which skite, clinopyroxene and minor amounts of amphibole. subsequently produce chains of volcanic islands as oceanic [4] Carefully selected whole-rock fractions (matrix free plates move over them. There is disagreement over which of exotic material) were crushed to less than 200 mesh and plume was responsible for the RSB basaltic activity. dissolved in a HF+HNO3 mixture. Rb-Sr, Sm-Nd and Pb According to one view, the Kerguelen hotspot that formed isotopic analyses followed standard chemical separation and the 90°E ridge and now located in the Indian Ocean mass spectrometric isotope dilution procedures [Anil Kumar (Figure 1) was responsible for this basaltic outpouring et al., 1999; Sarangi et al., 2003]. Total procedural blanks [Storey et al., 1992]. Curray and Munasinghe [1991] and during the course of these analyses were,<250 pg for Rb Subramanyam et al. [1999] suggested that the east Indian and Sr and <30 pg for Sm, Nd and Pb. magmatic province, the 85°E ridge was due to the Crozet plume, currently located beneath the Crozet plateau at 3. Results and Discussion 46.2°S(Figure1).Muller et al. [1993] matched the southern part of the 85°E Ridge (to 10°N) with the Conrad [5] Results of Sm-Nd and Rb-Sr isotopic measurements hotspot (53.4oS). Contrary to these plume links Anderson on the Jharia kimberlites are listed in Table-1 and Pb 1 et al. [1992] proposed that these Cretaceous lavas were isotopic data in Table-2 of supplement-1b . Initial Sr, Nd and Pb isotopic ratios calculated using an emplacement age Copyright 2003 by the American Geophysical Union. 0094-8276/03/2003GL018462$05.00 1 Auxiliary material available at ftp.agu.org/apend/gl/2003gl018462. SDE 9 -- 1 SDE 9 - 2 KUMAR ET AL.: KIMBERLITE FROM RAJMAHAL MAGMATIC PROVINCE 87 86 to the Kaapvaal Group II kimberlites ( Sr/ Sri = 0.7071 to 206 204 0.7109; eNd = À6.2 to À13.4; Pb/ Pbi = 17.06 to 17.63, 206 204 except Finish, Pb/ Pbi = 17.69 to 18.24, reviewed by Mitchell [1995]). This suggests distinctly different source compositions for the East Indian occurrences. 4. Comparison of Jharia Kimberlite With RSB and Indian Ocean Hotspot Lavas [7] In Figures 2a and 2b, the Sr vs. Nd and Sr vs. Pb isotopic data of Jharia kimberlites are compared with fields Figure 1. Physiographic elements of the Indian Ocean showing hotspots and their tracks. Location of Rajmahal, Sylhet and Bengal magmatic province (RSBP) is also indicated. of 116 Ma (supplement-1c) are illustrated in Figures 2a and 87 86 206 204 2b, on plots of eNd vs Sr/ Sri and Pb/ Pbi vs. 87 86 Sr/ Sri respectively. The initial Sr, Nd and Pb isotopic 87 86 ratios of the kimberlite ( Sr/ Sri = 0.70535 À 0.70561; 206 204 eNd = À2.6 to À3.2 and Pb/ Pbi = 17.88 to 18.07, 207 204 208 204 Pb/ Pbi = 15.51 to 15.60, Pb/ Pbi =38.00to 38.50) exhibit restricted variation and could reflect the composition of their mantle source, because minor amounts of contamination will not modify their isotopic composi- tions as these rocks have extremely high concentrations of Sr, Nd and Pb (Sr = 2203 to 3149, Nd = 249 to 397 and Pb = 39 to 47 ppm) in them. Their high 100 (Mg/(Mg + Fe) 87 86 values of 60–75 and Ni contents of 215–930 ppm (author’s Figure 2. (a) eNdT vs initial Sr/ Sr and (b) initial unpublished data) support this inference. 87Sr/86Sr vs 206Pb/204Pb plot of Jharia kimberlite data [6] Our kimberlite Sr, Nd and Pb isotopic ratios are similar (circular filled dots), Cretaceous Kerguelan plateau basalts to data reported on coeval lamprophyric sills from the Jharia (KE, Storey et al., 1989; Salters et al., 1992; Frey et al., 87 86 basin ( Sr/ Sri = 0.70491 to 0.70573 and eNd = À1.5 to 2002; Kent et al., 2002; Neal et al., 2002), Pristine À3.1; initial 143Nd/144Nd values were calculated using an Kerguelen plume (PKP) basalts [Weis et al., 1993, 1998], assumed Sm/Nd ratio of 0.12 for some of these samples, Rajmahal traps (RT I & II) [Mahoney et al., 1983; Storey et Rock et al. [1992]) and for one sample each from a minette al., 1992; Baksi, 1995; Kent et al., 1997), Crozet 87 86 and a lamproite dyke ( Sr/ Sri = 0.70596, eNd = À3.1, Archipelago basalts (CR) [Mahoney et al., 1996], Conrad 206 204 87 86 Pb/ Pbi = 17.82 and Sr/ Sri = 0.70493, eNd = À2.7, Rise lavas (CO) [Borisova et al., 1996], and Southeast 206 204 Pb/ Pbi = 17.82 respectively) occurring in the Ranigunj Indian Ridge (SEIR) [Mahoney et al., 2002]. Fields for basin about 50 km to their east [Middlemost et al., 1988]. On continental crust (CC) [Taylor and McLennan, 1985] and comparison (Figures 2a and 2b) of the Sr-Nd-Pd isotopic Kaapvaal Group II kimberlites (GIIK) [Mitchell, 1995] are compositions, the Jharia kimberlites possess distinctly lower also plotted. Numbers are DSDP sites. UG = upper group, 87 86 206 204 Sr/ Sri and higher eNd and Pb/ Pbi values compared LG = lower group. KUMAR ET AL.: KIMBERLITE FROM RAJMAHAL MAGMATIC PROVINCE SDE 9 - 3 87 86 for the RSB basalts [Mahoney et al., 1983; Storey et al., compositions ( Sr/ Sr À0.7046 to 0.7048; eNd +0.3 to 1992; Baksi, 1995; Kent et al., 1997], Cretaceous Kerguelen +0.9) but 206Pb/204Pb isotopic compositions (17.90 to plateau and Broken Ridge basalts [Frey et al., 2002; Kent et 17.98; Neal et al. [2002]) again overlap the kimberlite data. al., 2002; Neal et al., 2002], Cenozoic Kerguelen Archipel- Still younger basalts (85 Ma, Pringle et al.
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